Rhodium alloy coatings and method of making same



Patented Feb. 15, 1949 RHODIUM ALLOY COATINGS AND METHOD OF MAKING SAME Paul T. Smith, Esmond, and Joseph A. Smith,

Cranston, R. I.

No Drawing. Application December 24, 1947,

Serial No. 793,756

7 Claims. (Cl. 29-194) Our invention relates to improvements in the electrodeposition of alloy coatings upon articles and particularly relates to the electrodeposition of alloy coatings including rhodium. This is a continuation in part application of our abandoned application for Rhodium alloy coating and articles produced thereby, S. N. 409,317 filed September 2, 1941, which is a continuation in part of our abandoned application for Alloy coatings and articles produced thereby, S. N. 112,142, filed November 21, 1936. We have found that if a suitable electrolyte be prepared employing a soluble rhodium salt and a soluble metal salt of the cobalt nickel group and the electroplating takes place in a sufliciently acid bath that an alloy will be plated upon the article having the characteristics of .the metal of higher atomic weight therein in this instance rhodium, even if the proportion of said metal by weight to the metal of the nickel cobalt group is reduced to a proportion of substantially 20 of the nickel-cobalt group to 1 of rhodium. Thus a coating is obtained which if rhodium be employed is substantially acid proof .and at least'acid resistant, one which is tarnish resistant, bright, hard and substantially white, in other words, having in gen-- eral the characteristics of rhodium and in certain respects insofar as brightness and hardness is concerned being somewhat brighter than-the pure rhodium employed itself.

A further feature of our improved method is that it permits the use of electrolytes weaker in rhodium than heretofore thought possible and the plating from said electrolyte with standard electroplating equipment.

Exactly what takes place in this alloy electrodeposltion is not fully understood. It is possible that a nickelate and cobaltate might be formed, but it is believed that a double salt including the metal of the nickel or cobalt group as a base and rhodium also as a base is probably formed in the electrolyte which on the actual passage of current through the electrolyte splits off as a double cation comprising a molecule of rhodium and the metal of the nickel or cobalt group to plate on the cathode with a different alloy crystalline structure than is obtained from a direct plating of the rhodium or the metal of the nickel group,

said crystal when plated having more nearly the different and in many respects better characteristics than either. To provide a non-tarnlshable article with a flash outer alloy coating of our improved metal,

. we may if desired in accordance with the teachings of the prior art, preliminarily electrodeposit a wear resistant coating of substantial depth of tarnish resistant substantially white metal such as nickel, cobalt, or an alloy thereof prior to the electrodeposition of our improved alloy coating aforedescribed. This permits a substantially flash outer alloy coating, but it is apparent that substantially similar results may be obtained if the intermediate wear resistant coating be omitted and a longer thicker alloy electrodeposit be deposited upon the article in accordance with our invention.

In carrying out our improved process the following examples are given as typical, but are not attempted to be exclusive. The article may have a tarnishable metal body or outer surface, but usually comprises an outer surface which will readily receive the electrodeposit. The article may be cleaned if desired by an acid bath or otherwise prepared for electrodeposition in any well known manner in the art. As the initial wear resistant coating of tarnish resistant metal over the body we preferably employ a relatively thick electrodeposit of a metal of the nickel cobalt group and a suitable solution for this electrodeposition may be made up as follows:

Per gallon of water:

- Ounces Double nickel ammonium sulphate- (Ni804 (NHO2SO4 plus 6 H2O) 8 Nickel sulphate (NiSO4 plus 6Hz0) 4 Ammonium chloride (NH4C1) 2 Boric acid (H3303) 2 with a voltage ranging from 2 to 2.5 fora period of approximately five minutes, this will give a wear resistant coating of the desired thickness.

To provide the outer alloy coating 2. suitable bath may be made up as follows:

Per two gallons of water:

(3.36 ounces (13 grins.) by weight of metal of cobalt or and nickel in sulphate, chloride or any soluble form);

(.105 ounces (4. grm.) of metal of rhodium in sulphate, chloride orother soluble form.)

The solution is preferably neutralized by the addition of a solution of sodium carbonate or other suitable alkali and sufiicient C. P. sulphuric acid or orthophosphoric acid or any other suitable inorganic acid is added to acidity the solution to a low pH value not in excess of 3.5 (preferably around 1.7) and it has been found that 1 ounce C. P. sulphuric acid or 2 ounces of orthotwo gallons may be employed, and if commercial rhodium sulphate or phosphate be employed approximately one ounce of three gram metal solution per ounce may be employed per two gallons. If desired, at least 1 ounce of boric acid may be employed as a stabilizer. Thus an electrolyte was formed by this method containing per liter approximately 12.5 grams by weight of metal of cobalt and/or nickel in a solution of a soluble salt thereof and .4 gram by weight of metal of rhodium in the form of a soluble salt thereof per liter may be employed and to bring the pH value not in excess of 3.5 (preferably around 1.7) 4 c. c. of C. P. sulphuric acid were employed. We have found that cobalt, however, is slightly preferable to nickel as it gives a slightly brighter electrodeposit and will tend to brighten the under coating.

In the electrodeposition of articles with this method with a flash coating of our improved alloy, a current of five volts, an amperage of and a temperature of approximately 120 F. with an immersion of thirty seconds has successfully commercially alloy coated many gross of articles.

We have also found that our improved method readily lends itself to a continuous method and that if the electrolyte be prepared as hitherto stated and the pH kept down below 3.5, a true alloy plating of a metal alloy on said article will take place. In the particular electrolytes specifically mentioned as examples the approximate pH value of the electrolyte was actually around 1.7.

Example 2 Grams to 1 liter of water C0804.7H2O (12.6 gms. metal) "gms.... 60 Nat-S04 do- 7.5 HaBoa do 7.5 B11304 (.4 gm. metal) cc 4 H2804 do 2 phosphoric acid will be sufilcient for this pur- Example 3 Grams to 1 liter of water CoSo4.7H:O (12.6 gms. metal) --gms..- 60

Naasol do 7.5 HaBoa do 7.5 Rhflol (.4 gm. metal) -cc..- 4 H2504 do 2 A 3" x 1" brass strip was plated in this bath for 20 minutes with an amperage of 4 (96 amp/sq. ft.), a voltage or 5%. at a temperature of -150" 1'.

An analysis of this alloy plate showed that it contained- 98.3%, Co-1.7% Rh Example 4 Grams to 1 liter of'water CoSo4.7H:O (12.6 gms. metal) gms 60 Naasol do 7.5 H330: do 7.5 1311504 (.4 gm. metal) cc-- 4 H2804 do 2 A 3" x 1" brass strip was plated in this bath for 20 minutes with an amperage of 3% (84 amps/sq. ft.) a voltage of 5%, at a temperature of 135-150 F.

An analysis of this alloy plate showed that it A 3" x 1" brass strip was plated in this bath for 20 minutes with an amperage of 3 (72 amps/sq. ft.), avoltage of 4 /2, at a temperature of 135-150 1".

An analysis of contained- Example 6 Grams to 1 liter or water NlSoa'lHaO (13.32 gms. metal) ....gms-- 60 H330: do 7.6 RhSol (.4 gm. metal) ..cc 4 H2504 do 2 A 3" x 1" brass strip was plated in this bath for 20 minutes with an amperage of 3 (72 amps/sq. It.), a voltage of 4% at a temperature of 135-150 B.

An analysis of this alloy plate showed that it contained- 87% N1-13% Rh Example 7 Grams to 1 liter of water CoSorflHaO (3.15 gms. metal) gms- 15 NazSm do 7.5 HaBm do 7.5 RhSoa (.4 gm. metal) -cc-.. 4 H2804 do 2 A 3" x 1" brass strip was plated in this bath for 20 minutes with an amperage of 2% (60 amps./sq. ft.), a voltage of 6 at a temperature of 135-150 F.

this alloy plate showed that it enemas An analysis of this alloy plate showed that it contained- Example 8 Grams to 1 liter of water COSoa'lHzO (5.04 gms. metai)-.. ..-gms-- 24 HaBoa do 7.5

N52SO4 do- 7.5

Rhsoi (.4 gm. metal) "cc..- '4

H2804 do..-" 2

A 8" x l" brass strip was plated in this bath for 20 minutes with an amperage of 2% (60 amps./sq. ft. a voltage oi 6, at a temperature 01' 185-150 F.

An analysis of this alloy plate showed that it contained- Example 9 Grams to 1 liter of water C0S04.7H2O (4.2 gms. metal) ----gms 20 Naasol "dos-.." 7.5 HaBoa do 7.5 RhSOs (.4 gm. metal) cc 4 A H2804 ..do. 2

A 3" x 1" brass strip was plated in this bath for 20 minutes with an amperage of 2 (60 amp/sq. i't.), a voltage of 5% at a temperature of 135-150 F.

In Examples 2-9 the pH was measured with a quinhydrone and was around 1.7..

An analysis of this alloy plate showed that it contained 75.2 Co%-24.8% Rh We have discovered that the cathode current density affects the ratios of metals in the alloys plated from the same bath. thus as shown in Example 2 we employ a cathode current density of 60-amps. per square foot and as shown in Example 3 a cathode current density of 96 amps. per square foot. It will be noted that the greater the cathode current density, the larger relative amount of the metal of the nickel-cobalt group is plated out in the alloy.

As stated, We have found that our improved alloys stand a nitric acid test better than pure rhodium. Thus we plated a flash coat of the alloy given in Example 2 on a nickel base and applied a drop of nitric acid. The nitric acid took approximately 5 minutes to penetrate through.

platinum, lead, etc., or anodes alloyed in the same proportions as the metals in the bath.

Employingour improved process, articles have been continuously produced at a much less cost than articles finished with a flash'outer electrodeposit of rhodium having in certain respects qualities having more appeal to the trade. The pH's referred to herein have been measure. by a quinhydrone electrode.

It is understood that our invention is not limited to the specific embodiments shown or methods described and that various deviations may be made therefrom without departing from the spirit and scope of the appended claims.

What we claim is: 1. The method of alloy plating a .metal article with an alloy having the characteristics of the metal of higher atomic weight therein, comprising, immersing said article in an aqueous solution containing a soluble metal salt of the free inorganic acid to a value not less than 1 and not in excess of around 1.7 and passing current'through said bath with a cathode current density from about 40 to about 96 amperes per square foot and at a temperature from about 120 F. to about 150 F. to cause 'alloy plating of a metal alloy on said article with an alloy plate containing from 62-38 to 98.3-1.7%

by weight of the respective metals.

2. An article having a tarnishable metal body. a wear resistant coating of tarnish re sistant substantially white metal of at least one of the group consisting of nickel and cobalt over the body and an acid resistant, tarnish resistant, bright, hard, substantially white electrodeposited outer alloy coating consisting essentiallypf 95.4-98.3% by weight of metal of at least one of the group consisting, of nickel and 1 cobalt and 4.6-1.7% by weight of rhodium hav- We then applied a flash coat of pure rhodium to I a nickel base and applied a drop of nitric acid. Thispenetrated the flash coat in about thirty seconds, thus tending to prove that our improved alloy is ten times as resistant to nitric acid as the rhodium flash Hat employed. As stated above. we believe this is due to the fact that our improved alioy has a, much finer crystal than pure rhodium. Cur improved alloy may also be employed as a ease for a pure rhodium flash coat or thicker elcctrodeposit to provide a better acid ing characteristics of the metal of higher atomic weight, namely rhodium, therein.

3. An article having a metal body and a substantially acid-proof bright, hard, substantially white electrodeposited outer alloy coating consisting essentially of 95.4-98.3% by weight of metal of at least one of the group consisting of nickel and cobalt and 4.6-1.7% by weight, of rhodium, having characteristics of the metal of higher atomic weight, namely rhodium, therein.

4. An article having a tarnishable metal body.

a wear resistant coating of tarnish resistant substantially white metal of at least one of the group consisting of nickel .and cobalt over the body and an acid resistant, tarnish resistant, bright, hard, substantially white electrodeposited outer alloy coating consisting essentially 'of 75.2-98.3% 0! one of the group consisting of nickel and cobalt and 24.8-1.7% of rhodium, having characteristics of the metal of higher .atomic weight. namely rhodium. therein.

5. An article having a tarnishable metal body, a wear resistant coating of tarnish resistant substantially white metal of at least one of the group consisting of nickel and cobalt over the body and an acid resistant, tarnish resistant, bright, hard, posited outer alloy coating consisting essentially of 62-98396 of one of the group consisting of substantially white electrode-.

7 nickel and cobalt and 88-17% oi rhodium, having characteristics of the metal of higher atomic weight, namely rhodium, therein.

6. An article having a tarnishable metal body, and an acid resistant, tarnish resistant, bright, hard, substantially white electrodeposited outer alloy coating consisting essentially of 75.2- 98.3% of one or the group consisting of nickel and cobalt and 24.8-1.7% o1 rhodium, having characteristics of the metal oi higher atom weight, namely rhodium, therein.

7. An article having a tarnishable metal body, and an acid resistant, tarnish resistant, bright, hard, substantially white electrodeposited outer alloy coating consisting essentially of at least 62-98.3% or one of the group consisting of nickel and cobalt and 384.7% of rhodium, having characteristics of the metal of higher atomic weight, namely rhodium, therein.

PAUL T. SMITH. JOSEPH A. SMITH.

The following reierences are of record in the file 01' this patent:

UNITED STATES PATENTS 3 

